Monitoring of Terrorism Threats Has Risen, Official Says

WASHINGTON — A senior European counterterrorism official said on Thursday that spy services in several countries had increased their monitoring and surveillance, and governments had put heightened security measures in place, even before recent arrests in Belgium and Turkey.

Hours after the official spoke, the police in the southern German city of Munich evacuated two train stations and warned residents to avoid large groups of people, citing “concrete hints” of a possible terrorist attack amid New Year’s celebrations.

Joachim Herrmann, interior minister for the state of Bavaria, of which Munich is the capital, told reporters early Friday that the German authorities had been tipped by a foreign intelligence service that the Islamic State was linked to a plot to carry out attacks in Munich.

Hubertus Andrä, head of the Munich police, said officials suspected that several suicide bombers had planned to carry out the attacks.

The specific nature of the threat and the number of people on the streets for the holiday led the authorities to act, Mr. Herrmann said.“Our standard line is not to let terrorists dictate our life,” he added. “The threat was specifically targeting midnight and these two locations.”

Extra precautions had already been in place across Europe after the Austrian police said last weekend that they had received a warning from a “friendly” intelligence service about possible terrorist attacks by the Islamic State in several European countries, including Austria, Belgium, France and the Netherlands.

“It was a general warning about possible attacks against soft targets,” said the counterterrorism official, who spoke on condition of anonymity to discuss confidential assessments.

Senior security officials in Washington and in European capitals said that American spy agencies had played a major role in detecting possible plots in the days leading up to the year-end holidays and in sharing that information. They said the information had come from eavesdropping on suspected Islamic State terrorists and from social media.

“A lot of that of information has led to many of the arrests we’ve seen in the past week,” said a senior American official, who also spoke on condition of anonymity to discuss confidential terror assessments.

Shortly before midnight, the police evacuated Munich’s main train station — where thousands of mostly Syrian refugees were greeted this year — and a second station in the Pasing district.

There were no arrests, Mr. Andrä said.

The Bavarian authorities said they had received hints of possible threats to public places over the past week, none of which were considered serious enough to require action.

Since the Paris attacks in November, the senior European official said, security analysts across Europe, as well as in the United States, have been sharing more information as they receive it. In the past, spy services typically waited longer to develop tips more fully before passing them on to other countries.

“What the Paris attacks taught us were that a lot of weak intelligence signals could be connected earlier,” the official said. “Now the thinking is, don’t wait, we don’t want to miss an opportunity.”

Several officials said that online terrorist chatter had increased in the days leading up to New Year’s Day, but that it was difficult to sort out which plots being discussed were credible, prompting officials to take measures like the recent arrests.

Two senior American counterterrorism officials said on Thursday that there were no credible indications of terrorist plots against the United States or American interests abroad, but warned that terrorists’ increasing use of encrypted communications has made it more difficult to determine details of plots.

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